More Detail On The Multiyear Aftereffects Of ENSO – Part 1 – El Nino Events Warm The Oceans

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This is part 1 of a multipart post. It addresses critical comments about my earlier posts that dealt with the multiyear aftereffects of significant traditional El Nino events. Two specific El Nino events, those in 1986/87/88 and the 1997/98, caused Sea Surface Temperatures of the East Indian and West Pacific Oceans to remain at elevated levels during the subsequent La Nina events. These SST residuals, what I have called step changes in earlier posts for the sake of simplicity, bias the global SST during the La Nina events and give the impression of a gradual increase, one that is attributed to anthropogenic greenhouse gases.

This first post in the series is for those who wanted confirmation in scientific papers that El Nino events warm the oceans remote to the Pacific through processes other than heat transfer. In this post, I’ll use two papers as reference.

Wang (2005) describes how sea surface temperatures can and do rise in response to El Nino events in areas of the global oceans remote to the tropical Pacific. It concentrates on the Tropical North Atlantic and the Western Hemisphere Warm Pool. Basically, the rise in sea surface temperature of the North Atlantic is a response to changes in atmospheric circulation. Wang (2005) is provided here for those familiar with Hadley and Walker Circulation, wind stress and the like.

In the Abstract Wang (2005) writes, “The chapter also discusses a tropospheric bridge by the Walker/Hadley circulation that links the Pacific El Niño with warming of the tropical North Atlantic (TNA) and the WHWP.” For those who want the details, he provides the detailed discussion in subchapter 8 on page 22. His Summary and Discussion includes, “ENSO shifts the western Pacific heat source and atmospheric convective activity and then affects global atmospheric circulation. During El Nino, the equatorial Pacific Walker circulation is observed to be weakened. The anomalous meridional Hadley circulation in the eastern Pacific shows the air rising in the tropics, flowing poleward in the upper troposphere, sinking in the subtropics, and returning to the tropics in the lower troposphere. The anomalous Hadley circulation in the western Pacific is opposite to that in the eastern Pacific, indicating a weakening of the western Pacific Hadley circulation during El Nino. The NCAR/NCEP reanalysis field also shows that El Niño weakens the Atlantic Hadley circulation, consistent with an earlier result of Klein et al. (1999) that is inferred from correlation maps of satellite observations, and with the direct circulation analyses of Mestas-Nunez and Enfield (2001) and Wang (2002a). Wang (2002b, c) and Wang and Enfield (2003) suggest that following El Nino winters in which the Atlantic Hadley circulation is strongly weakened, the decreased subsidence over the subtropical North Atlantic results in the late winter weakening of the NE trades off Africa, the associated spring TNA warming (Enfield an Mayer 1997 and others), and the large summer warm pools (Wang and Enfield 2001).”

Again, Wang (2005) explains how El Nino events can and do raise SST in an area remote to the tropical Pacific. The response of the North Atlantic to ENSO can also be seen in a comparison graph of NINO3.4 SST anomalies and North Atlantic SST anomalies, Figure 1.

Note: The North Atlantic, of course, is also impacted by the AMO, which imposes an additional increase in SST anomalies over the term of the OI.v3 SST dataset. Refer to Figure 2. The impact of the AMO can be seen in a comparison graph of North Atlantic SST anomalies linear trends and the SST anomalies linear trends of the other ocean basins. This is discussed further in my post “Putting The Short-Term Trend Of North Atlantic SST Anomalies Into Perspective.”

In it, Trenberth et al provide broader discussions of how ENSO events can and do impact global LST and SST. The paper deals with the period of 1950 to 1998, which obviously will not include the multiyear aftereffects of the 1997/98 El Nino that is evident in the data, but it does reinforce Wang (2005). It is an excellent reference for those interested in ENSO dynamics and global responses to ENSO.

In the abstract, Trenberth et al write, “However, most of the delayed warming outside of the tropical Pacific comes from persistent changes in atmospheric circulation forced from the tropical Pacific. A major part of the ocean heat loss to the atmosphere is through evaporation and thus is realized in the atmosphere as latent heating in precipitation, which drives teleconnections. Reduced precipitation and increased solar radiation in Australia, Southeast Asia, parts of Africa, and northern South America contribute to surface warming that peaks several months after the El Nino event. Teleconnections contribute to the extensive warming over Alaska and western Canada through a deeper Aleutian low and stronger southerly flow into these regions 0–12 months later.”

In other words, there are El Nino-induced processes other than heat transfer that cause warming outside of the tropical Pacific.

Back to the oceans: Trenberth et al go further under the heading of “3.3. Evolution of Spatial Patterns” to document the lag correlations between NINO3.4 SST anomalies and the SST anomalies of the Atlantic, Pacific, and Indian Oceans. In other words, they illustrate the length of time required for the major ocean basins to respond to the El Nino event. I have reproduced the Trenberth et al Figure 7 in this post as my Figure 3. They write, “Figure 7 shows a breakdown of Figure 3 by ocean sector for 1950–1978 and 1979–1998. It seems obvious that the lag of surface temperatures in the Pacific should be closely in phase with N3.4 because of the close proximity, and indeed this is the case, with a 2-month lag in both subperiods. However, the peak correlation is almost doubled in the earlier period. The Atlantic sector (defined as 90W–0) lags by 4–5 months, while the Indian Ocean sector (defined as 0–120E) shows a lag of 7 months for 1979–1998 but a skewed and smaller lag centered around +5 months from 1950 to 1978.”http://i35.tinypic.com/2mzfbbm.jpg
Figure 3

Throughout the rest of the paper they discuss the processes that cause the Atlantic, Pacific, and Indian Oceans to respond to El Nino events. These descriptions and discussions make up the body of the paper, so it’s not practical to reproduce all of it here.

With respect to heat transfer, in the Discussion Section, page 13, first subsection, Role of the Tropical Pacific Ocean, paragraph 37, Trenberth et al further describe, “The evolution of ENSO in the tropical Pacific Ocean illustrated here supports much of that previously described by Barnett et al. [1991], Zhang and Levitus [1996], Tourre and White [1995], Giese and Carton [1999], Smith [2000], and Meinen and McPhaden [2000] in the way that anomalies of subsurface ocean heat content in the western Pacific develop as they progress eastward across the equatorial Pacific, often with a dipole pattern across the Pacific, and then with anomalies progressing off the equator to higher latitudes. Zhang and Levitus [1996] found links only to the North Pacific, perhaps reflecting the available data, while our results reveal strong links to both hemispheres. The SST evolution lags somewhat behind that of the subsurface ocean and is damped by surface fluxes and transports out of the region by the atmosphere, emphasizing the dominant role of the surface wind stress and ocean dynamics and advection in producing the local ocean heat content and SST anomalies. This damping of the ocean signal, however, forces the atmospheric anomalies. Moreover, this aspect also emphasizes that in cold La Nina conditions the surface fluxes of heat are going into the ocean relative to the mean and are warming the ocean, although not locally as the heat is redistributed by currents.”

There are many more discussions of the diabatic and adiabatic processes throughout the paper. But El Nino events do cause SST anomalies outside of the tropical Pacific to rise. These can be seen in comparison graphs of NINO3.4 SST anomalies and the SST anomalies of the ocean basins, Figures 4 through 7. The North Atlantic comparison is shown above In Figure 1.

Through changes in atmospheric circulation and through the redistribution of warm water by ocean currents, El Nino events cause SST anomalies to rise.

The next post is the discussion of ENSO discharge/recharge. Trenberth et al provide an overview of this on page 16, paragraph 57, for those who want to read ahead. That post will also reinforce the processes that cause the SST anomalies of the East Indian and West Pacific to linger through the La Nina events and cause the bias that is mistaken for anthropogenic warming of the oceans.

9 comments:

Anonymous
said...

BobI saw this in the CRU hack dossier and thought of you:

"The CERES data published in the August BAMS 09 supplement on 2008 shows there should be even more warming: but the data are surely wrong. Our observing system is inadequate. That said there is a LOT of nonsense about the PDO. People like CPC are tracking PDO on a monthly basis but it is highlycorrelated with ENSO. Most of what they are seeing is the change in ENSO not real PDO. It surely isn't decadal. The PDO is already reversing with the switch to El Nino. The PDO index became positive in September for first time since Sept 2007. seehttp://www.cpc.ncep.noaa.gov/products/GODAS/ocean_briefing_gif/global_ocean_monitoring_current.ppt"

"Saying it is natural variability is not an explanation. What are the physical processes?

Where did the heat go? We know there is a build up of ocean heat prior to El Nino, and a discharge (and sfc T warming) during late stages of El Nino, but is the observing system sufficient to track it?Quite aside from the changes in the ocean, we know there are major changes in the storm tracks and teleconnections with ENSO, and there is a LOT more rain on land during La Nina (more drought in El Nino), so how does the albedo change overall (changes in cloud)? At the very least the extra rain on land means a lot more heat goesinto evaporation rather than raising temperatures, and so that keeps land temps down: andshould generate cloud. But the resulting evaporative cooling means the heat goes into atmosphere and should be radiated to space: so we should be able to track it with CERES data. The CERES data are unfortunately wonting and so too are the cloud data. The ocean data are also lacking although some of that may be related to the ocean current changes and burying heat at depth where it is not picked up

If it is sequestered at depth then it comes back to haunt us later and so we should know about it."

In the latest ENSO heatmap (http://www.ecmwf.int/products/forecasts/d/charts/ocean/real_time/xzmaps/), I see the cold anomalies along the coast are still there, and seem to be in fact increasing (for now, who knows what will happen in the coming weeks). I mentioned that as something interesting to watch in a prior post, and it continues to be - it looks to have basically stalled the heat bubble's movement east.

Is that cold anomaly caused by the humboldt current coming up from S. America? I'm trying to figure out how it can remain just as cold, if not colder, over time with that much warm water trying to spread eastward. Just curious where that tiny slip of cold water is coming from, so I can better understand the process.

John: You asked, "Is that cold anomaly caused by the humboldt current coming up from S. America?"

Using the link you provided, switch the view to "full field" so that you're looking at subsurface temperatures, not anomalies. As the Kelvin wave continues east and the thermocline flattens, the cooler water along the coast should be replaced with the warmer water. We shall see.

Good - the more I think about 1998, the more I think you are on to something.

I'll keep my eyes peeled.

Can you point me to a good read about OHC - I'm specifically curious about how do they weight surface areas for SST and volumes for OHC when combining seas and oceans into global figures. It's only a idle curiosity.

Anonymous: You asked, "Can you point me to a good read about OHC - I'm specifically curious about how do they weight surface areas for SST and volumes for OHC when combining seas and oceans into global figures. It's only a idle curiosity."

I looked into this a while back. I know I started with Levitus et al (2000):http://www.sciencemag.org/cgi/content/abstract/287/5461/2225

Then I used the references and google to go from there. I don't recall where I wound up, but there are a few papers that go into detail about the temperature measurements at depth, salinity measurements, etc. It's likely that this is also discussed in more recent papers.

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